Adaptive Growth Responses to Osmotic Stress of Hypocotyl Sections of Vigna unguiculata: Roles of the Xylem Proton Pump and IAA

Abstract

The growth responses to osmotic stress of hypocotyl sections of Vigna unguiculata were studied by the xylem perfusion method. Hypocotyl sections shrank upon exposure to osmotic stress. Sections showed no adaptive responses to osmotic stress when they were in an IAA-depleted condition as a result of perfusion with solutions that lacked IAA for 3–4 h. The correlation between the growth rate and the membrane potential of the xylem/symplast boundary (Vpx) was very limited in the absence of IAA. By contrast, hypocotyl sections showed distinct adaptive responses to osmotic stress after perfusion with solutions that contained 10 μM IAA. In the presence of IAA, Vpx increased in the negative direction and growth resumed in spite of the osmotic stress. The growth rate was closely correlated with the xylem membrane potential. Hyper-polarization of the membranes of the xylem/symplast boundary always preceded the recovery of growth under osmotic stress. It appears that IAA is essential for the adaptive recovery of growth under osmotic stress and, moreover, that the xylem proton pump plays an indispensable role in modulating the growth of hypocotyl sections. This result confirms prediction of an earlier simulation study using the apoplast canal model [Katou and Furumoto (1986) Protoplasma 133: 174, Katou and Enomoto (1991) Plant Cell Physiol. 32: 343].